This invention relates to Magnetic Resonance Imaging (MRI) systems. More particularly, this invention relates to a quench protection circuit.
As is known, a coil wound of superconductive material (a magnet coil) can be made superconducting by placing it in an extremely cold environment. For example, a coil may be made superconducting by enclosing it in a cryostat or pressure vessel containing a cryogen. The extreme cold enables the superconducting wires to be operated in a superconducting state. In this state, the resistance of the wires is practically zero. To introduce a current flow through the coils, a power source is initially connected to the coils for a short time period. In the superconducting state, the current will continue to flow through the coils, thereby maintaining a strong magnetic field. In other words, because superconductive windings offer no resistance to electrical current flow at low temperatures, the superconducting magnet is persistent. The electric current that flows through the magnet is maintained within the magnet and does not decay noticeably with time. Superconducting magnets have wide applications in the field of magnetic resonance imaging (“MRI”).
In a typical MRI magnet, the main superconducting magnet coils are enclosed in a cylindrically shaped cryogen pressure vessel. The cryogen vessel is contained within an evacuated vessel and formed with an imaging bore in the center. The main magnet coils develop a strong magnetic field in the imaging volume of the axial bore.
A common cryogen is liquid helium. During superconducting operation, liquid helium boils to form helium gas. The gas is either recondensed for recycling and reuse or is vented to the atmosphere.
One concern in such apparatuses is the discontinuance of superconducting operation (i.e., “quenching”). Quenching can produce undesirable and possibly damaging high temperatures and voltages within the magnet. During a quench event, the current in the persistent superconducting coils decays rapidly. The rapid decay is as a result of resistive zone(s) developed in the coils for example due to thermal or mechanical disturbance. Quenching may occur from an energy disturbance, such as from a magnet coil frictional movement. The energy disturbance heats a section of superconducting wire, raising its temperature above the critical temperature of superconducting operation. The heated section of wire becomes normal (i.e., non-superconducting) with some electrical resistance. The resulting I2R Joule heating further raises the temperature of the non-superconductive section of wire increasing the size of the non-superconductive section. An irreversible action called quench then occurs. During a quench, the electromagnetic energy of the magnet is quickly dumped or converted into thermal energy through the increased Joule heating.
For MRI applications, a homogeneous magnetic field is desired in the imaging volume. To provide the desired homogeneity, the magnet coil is divided into a plurality of coils. These coils are spaced along and around the axis of the superconducting magnet such that they are not thermally connected. As a result, when only one of the superconducting coils quenches, the entire energy of the strong magnetic field may be dumped into the quenching coil. A hot spot and possible damage is caused unless a suitable quench protection system is provided. Quench protection can be accomplished by quickly quenching the other coils. Damage from a rapid rise in temperature and voltage, or from a quick electromagnetic force change in the magnet, is thereby prevented. A number of quench systems for protection of superconducting magnets are known. For example, U.S. Pat. No. 6,147,844 to Huang et al, U.S. Pat. No. 5,739,997 to Gross, and U.S. Pat. Nos. 5,650,903 and 5,731,939 to Gross et al. relate to quench protection circuits for superconducting magnets.
An actively shielded MRI magnet consists of main coils and shielding coils (also herein referred to as bucking coils). The main and shielding coils produce a homogeneous field in the image volume and reduce fringe fields. Most of the superconducting MRI magnets are made of coils that are symmetric with respect to a symmetry mid-plane. During a quench, the current in different coils may not decay at exactly the same rate. A net differential force acting on the coils and coil supporting structure could thereby be induced. The net differential force in the right and left halves of the magnets and/or between the main coil and the bucking coil structures results in an unbalanced quench. An unbalanced quench could potentially damage coil supporting structures, and thermal shields depending on the severity of the unbalance.
Due to the exact same reason that the current in different coils may not decay at exactly the same rate, the shielding magnetic field produced by the bucking coil(s), and the main magnetic field produced by the main coil(s) may not be canceling each other exactly, resulting in a temporary fringe field blooming, which is a phenomenon where the static fringe field lines, such as a 0.5 mT line, extends temporarily beyond the normal operation limit. Fringe field blooming is undesirable.